1. Field of the Invention
The present invention relates to a semiconductor laser device having a plurality of semiconductor laser diodes connected in series, and a semiconductor-laser excited solid-state laser apparatus having this semiconductor laser device.
2. Description of the Related Art
In production controls (e.g., stock control, process control, quality control, etc.) which are carried out in a company or so, so-called marking or a work of marking characters, a figure, a symbol or so on products, parts, packages and so forth is often performed.
The marking is carried out by various ways, such as printing, ink jet, stamping, sealing, engraving, punching, edging, ultrasonic wave and laser. Of those schemes, laser marking using a laser beam has been receiving attention recently due to its multiple features, such as non-contact, fineness and delicateness, non-vanishing marking, dry process, fast process, easy automation, easy power saving and flexibility.
Laser marking is executed by using a laser marker which uses a CO2 laser or gas laser, a YAG laser or a solid-state laser, or the like.
According to the exciting system, there are different types of YAG lasers that include a lamp-excited YAG laser which uses a discharge tube and a semiconductor-excited YAG laser which uses a semiconductor.
Because the semiconductor excitation has a higher energy conversion efficiency from electric energy to optical energy by about 30% to 40% and a very short emission spectrum width of about 3 mm, as compared with the lamp excitation, it provides an extremely high efficiency when matched with the absorption spectrum of the YAG crystal. Further, the semiconductor excitation has an excellent directivity to achieve easier light condensing.
While the YAG laser, like a ruby laser and glass laser, belongs to a solid-state laser, a solid-state laser which has a semiconductor as an excitation light source (LD-excited solid-state laser), particularly, has features such as high out power, high efficiency, high beam quality, small size, high stability and long life.
Accordingly, the LD-excited solid-state laser is recently used in every field, such as laser processing on electronics (trimming and fine processing, thin film generation, etc. by a marking apparatus), physical and chemical research (spectral analysis), welding of vehicle bodies, medical care, aeronautics, biotechnology, life science and engineering.
Particularly, an LD-excited solid-state laser apparatus which emits laser power of kilowatts using over hundred LDs in a single apparatus has been developed.
There are mainly a rod type, fiber type and thin disk type LD-excited solid-state lasers.
In the rod type, as shown in FIG. 1, with excited lights from a laser array 110, a laser rod 120 excites a laser beam which reciprocates between a rear mirror 130 and an output mirror 140 and is output from the output mirror 140.
The laser array 110 has a plurality of semiconductor laser diodes to irradiate several ten to several hundred lights onto the laser rod 120.
This semiconductor laser diode is said to have a longer life more than ten times the life of a lamp and be continuously usable for 10,000 hours or so.
As this is an average time, however, some of the semiconductor laser diodes may suffer power reduction in several thousand hours or so and it is difficult to completely identify and remove them in the initial selection of semiconductor laser diodes.
The semiconductor laser diode that is degraded and fails to output light becomes a heat source which generates heat twice as great as the heat generated by a normal semiconductor laser diode. Such a semiconductor laser diode damages itself or parts surround it and eventually results in disconnection.
A semiconductor laser diode may have its life shorted considerably by disturbance, such as an electric surge from static electricity or a power source, return light, dust or gas, or dewing, or an external environment not only due to a variation in its components or its productional variation but also depending on the state where the semiconductor laser diode is mounted on a semiconductor laser device, and may be damaged or disconnected by abnormal heat generated.
What is more, plural semiconductor laser diodes in a semiconductor laser device are connected in series to one another, so that when a single semiconductor laser diode is disconnected due to failure, for example, the current does not flow to the other semiconductor laser diodes.
That is, failure or disconnection of only a single semiconductor laser diode in several ten to several hundred semiconductor laser diodes disables the other multiple semiconductor laser diodes to output lights. This inhibits emission of a laser beam from an LD-excited solid-state laser apparatus 100.
In this respect, various schemes have been proposed by which even if at least one semiconductor laser diode in a plurality of semiconductor laser diodes fails or is disconnected, other normal semiconductor laser diodes can be enabled to emit lights continuously.
For instance, there is a circuit which has a plurality of series-connected light emitting elements to each of which one Zener diode or at least two Zener diodes are connected (as disclosed in, for example, Japanese Utility Model Laid-Open No. 088499/1985 which is referred hereafter as first prior art).
There is another example that has a monitor circuit which monitors the optical outputs of a plurality of semiconductor laser diodes, a comparison circuit which determines a drop in each optical output bypass diode a detection signal from the monitor circuit and a bypass control circuit which causes the current to flow to a bypass circuit, not the semiconductor laser diodes, when the optical output has dropped (as disclosed in, for example, Japanese Patent Laid-Open No. 284789/1998 which is referred hereinafter as second prior art).
As the first prior art uses the reverse voltage characteristic (breakdown characteristic) of a Zener diode as a bypass device, however, it has a problem on control of the Zener diode.
In general, a laser processing apparatus using a Zener diode is demanded of a quicker rising time for the laser output to become a rated level and is used in many ways to modulate the laser output in an arbitrary waveform. The laser processing apparatus therefore employs feedback control of the current from a laser diode while monitoring the laser beam.
In such a feedback system, it is very important to set the response time constant of a circuit which includes a laser diode. A diode and a Zener diode significantly differ from each other in current-voltage characteristic. In case where some diode in the circuit is disconnected and the current starts flowing to a Zener diode connected in parallel to the diode, therefore, the time constant of the circuit that is set with diodes alone greatly varies, which may lead to a significant delay of the rising time, a dull waveform or overshooting.
Individual phenomena originated from such a change in time constant delay the recovery time after disconnection and a variation in the output intensity of the laser beam in a semiconductor laser device which enables continuous light emission even if some diode is disconnected, and should therefore be overcome.
As the fist prior art uses an electric bulb or fluorescence lamp as a light emitting element, the breakdown characteristic obtained when the reverse voltage is applied to a Zener diode can be used. In case where a semiconductor laser diode is used as a light emitting element, however, the breakdown characteristic of the Zener diode cannot used sufficiently.
When a light emitting element is an electric bulb or fluorescence lamp, for example, the voltage and the current to cause the light emitting element to emit light are normally several tens of volts to a hundred and several tens of volts and several hundred milliamperes to several amperes, respectively.
By way of contrast, a Zener diode is normally driven when applied with a voltage of over 5 volts and the general rated current ranges from several hundred milliamperes to several amperes.
In case where an electric bulb or fluorescence lamp is used as a light emitting element, therefore, the use of a Zener diode as a bypass device is the usage within the range of the rated values so that it is possible to make good use of the breakdown characteristic.
In case where the light emitting element is a semiconductor laser diode, however, the voltage and the current to cause the semiconductor laser diode to emit light are normally 1 to 3 volts and around hundred amperes, respectively.
Within the range of a low voltage (approximately 5 volts or lower), it is normally difficult that the Zener diode provides a stable voltage in a low current range.
In case where a laser diode is used as a light emitting element, therefore, the use of a Zener diode as a bypass device raises a problem that a stable breakdown characteristic cannot be acquired due to a low voltage being applied to the Zener diode.
Because the second prior art has the monitor circuit, comparison circuit, etc. in addition to the bypass circuit, it has a problem that space for the individual circuits should be secured.
Recently, attempts have been made to make a semiconductor laser device and an LD-excited solid-state laser apparatus compact to enhance the functionality and portability. This requires that a circuit which enables continuous light emission even if at least one semiconductor laser diode fails or is disconnected should also have a simple and compact circuit structure.